An as­ter­oid whose fiery encounter with our atmo­sphere in­cin­er­at­ed a swath of Si­be­ri­an for­est, in 1908, was only a frac­ti­on as large as pre­vi­ously thought, new re­search sug­gests. That would mean such dev­as­tat­ing events are more com­mon than es­ti­mat­ed be­fore—and might hap­pen eve­ry two to three cen­turies, if a re­cent NASA anal­y­sis is cor­rect.

Re­search­er Mark Bos­lough points out de­tails of a "fire­ball" that a sim­u­la­tion shows would re­sult from an as­ter­oid burst­ing in the at­mos­phere. (Pho­to by Randy Mon­toya)

As with ordi­nary rocks, small as­ter­oids out­num­ber big ones. Thus events like the Si­be­ri­an blast “are not as im­prob­a­ble as we had be­lieved,” said Mark Bos­lough of San­dia Nati­onal Lab­o­r­a­to­ries in New Mex­i­co, prin­ci­pal in­ves­ti­ga­tor in the new stu­dy. “We should be mak­ing more ef­forts at de­tect­ing the smaller ones.”

The June 30, 1908 ex­plo­si­on over Tun­gus­ka, Si­be­ria, flat­tened some 60 mil­li­on trees over about 2,000 square km (500,000 acres) of un­pop­u­lated for­est.

Bos­lough and col­leagues ran su­per­com­puter si­m­ul­a­tions to re-enact what hap­pens when an as­ter­oid bursts in the sky, as is be­lieved to have oc­curred then. The sim­ul­a­tions—which the U.S. go­vernment lab­o­r­a­to­ry con­tends are the best to date—show the blast shoot­ing a hot fire­ball down­ward faster than sound.

Com­pared to an ex­plo­si­on that stays where it started, Bos­lough said, this causes stronger blast waves and blind­ing light pulses, or ther­mal ra­di­a­tion, at the ground lev­el. “Our [pre­vi­ous] un­der­stand­ing was over­sim­p­li­fied,” he said. Now, “we no long­er have to make the same sim­pli­fy­ing as­sump­ti­ons, be­cause pre­s­ent-day su­per­com­puters al­low us to do things with high res­o­lu­ti­on in 3-D.”

The re­search of­fers one en­cour­ag­ing rev­el­ati­on, Bos­lough said: the blast caused “less dev­ast­a­tion than pre­vi­ously thought,” be­cause ex­tra­ne­ous fac­tors likely am­pli­fied the dam­age. For one, for­esters be­lieve the af­fect­ed wood­land was un­healthy at the time. None­the­less, Bos­lough and col­leagues said, the most im­por­tant find­ing was the small size of the as­ter­oid, and this should prompt a new look at cur­rent as­ter­oid-de­tec­tion ef­forts.

What really de­ter­mines an as­ter­oid’s de­struc­tive po­ten­tial, he wrote in an e­mail, is its ki­net­ic en­er­gy, or mass times ve­lo­city squared.

But the new find­ings in­di­cate that for a giv­en speed—es­ti­mat­ed to have been more than five times that of sound, in this case—the as­ter­oid needed to be only one-third to one-fourth as heavy as pre­vi­ously es­ti­mat­ed. “If the White House were made out of a sol­id rock, it would have just about the right mass,” Bos­lough wrote; so would a stone ball 30 to 40 me­ters (33 to 44 yards) wide. The nec­es­sary size al­so de­pends on the ob­jec­t’s pre­cise make­up, added Bos­lough, who with col­leagues drew world­wide at­ten­ti­on in the 1990s by cor­rectly pre­dict­ing that com­et Shoemaker-Levy 9 would cre­ate a fire­ball vis­i­ble from Earth when it hit Ju­pi­ter.

The new Tun­gus­ka sim­ul­ati­ons in­di­cate that an in­com­ing as­ter­oid is com­pressed by re­sist­ance from Earth’s at­mos­phere. As it pushes deeper, the grow­ing re­sist­ance blasts it apart, lead­ing to the down­ward flow of hot gas. Be­cause this en­hances the de­struc­ti­on, the blast probably needed to re­lease only three to five mega­tons of pow­er, the re­search­ers said—not the pre­vi­ously es­ti­mat­ed to 10 to 20.

Such a re-evalu­ati­on of the pow­er would mean Tun­gus­ka-type events hap­pen around three times as of­ten as pre­vi­ously be­lieved, ac­cord­ing to a 2003 re­port by NASA’s Near-Earth Ob­ject Sci­ence Def­i­ni­ti­on Team. Spe­cif­ic­ally, the re­port said, such re­vised es­ti­mates would sug­gest Tun­gus­ka-type events hap­pen eve­ry 250 years or so on aver­age, rath­er than eve­ry 600 to 1000.

Bos­lough’s work was pre­sented at the Amer­i­can Geo­phys­i­cal Un­ion meet­ing in San Fran­cis­co on Dec. 11, and ac­cept­ed for pub­lic­ati­on in the In­tern­ati­onal Jour­nal of Im­pact En­gi­neer­ing.